Super Kamiokande

. This neutrino detector has been built a kilometer underground in a Japanese zinc mine. The huge stainless-steel vessel, 40 meters tall and 40 meters wide, has been filled with 50,000 tons of highly purified water. Its walls are lined with 11,000 light sensors, called photo-multiplier tubes, that pick up a flash of light generated by electrons recoiling from neutrino collisions in the water. (Courtesy of Yoji Totsuka, Institute for Cosmic Ray Research, University of Tokyo.)